Here's a clue: paid shills generally do not announce that they are being paid by the entity they are allegedly shilling for.
All the linked article is reporting is that an analyst has taken a job to write a report--which is known because the analyst posted about it on his blog, naming the client and what the report is about, and stated that he would post his findings.
Under your ridiculous apparent definition of "shill", EVERYONE who does any research for pay is a shill for whoever is paying them.
edit: wow. Downvoted for calling out an unsupported ad hominem.
Muller is notorious for taking anti-Google positions, and his anti-Google stance appears to be so extreme it affects his judgement.
Meanwhile, he is getting paid by Microsoft unknown amounts of money for unknown work - it's not just a single report. To quote:
He says that he and Microsoft have worked together more than once, but that because of a confidentiality agreement with the company, he can’t elaborate on the details of that relationship
A further quote from that article is interesting:
Mueller may have perfectly good reasons to consistently zing Android—he’s far from the only pundit that has consistently strong views about a particular company. But given his financial relationship with that company’s archenemy—and the fact that Microsoft has a history of “hiring” outside experts to attack the competition—it’s hard to regard him as a disinterested party. The time has come for Mueller to amend the conflict of interest disclaimer on his blog and for the media to cease citing him as an impartial authority.
This post has more on the story and Mueller has not denied it's veracity:
"He says that he and Microsoft have worked together more than once, but that because of a confidentiality agreement with the company, he can’t elaborate on the details of that relationship."
Mueller buries his bias and sensationalism amongst otherwise insightful analysis. He also offers no opportunity for rebuttal on his blog (comments are always disabled).